1. Field of the Invention
The invention relates to tunable lasers, in particular, tunable semiconductor lasers.
2. Discussion of the Related Art
Tunable laser sources are useful in a variety of fields, including telecommunications, medicine, material diagnostics, spectroscopy, isotope separation and remote sensing. In applications such as optical sensors, as well as dense wavelength-division-multiplexed (DWDM) optical communication systems, which are growing in importance due to their ability to transmit large amounts of data, there is an increasing demand for laser sources capable of being tuned and/or chirped over a relatively wide range of wavelengths. Such tunable lasers offer the potential for substantially increasing the transmission capacity of multi-wavelength optical networks by providing different wavelengths on demand, as well as the potential for improving network reliability by serving as backups for sets of fixed-frequency lasers.
Several types of lasers capable of tuning are currently in use. (See, e.g., J. F. Ready, Industrial Applications of Lasers 2.sup.nd Ed., Academic Press (1997); and Tunable Lasers Handbook, F. J. Duarte, ed., Academic Press (1995).) These lasers include dye lasers, semiconductor lasers, optical parametric oscillators (OPOs), and free electron lasers (FELs). Of these, semiconductor lasers offer numerous advantages. Dye lasers and OPOs require pump lasers in order to operate, while the FELs are relatively large and very expensive. The Group III-V semiconductor lasers, which are useful in the red and near infrared region of the optical spectrum, do not have these drawbacks. In addition, semiconductor lasers have relatively high efficiency, small size, low weight, low power consumption, and the capability to be driven by low-voltage power supplies. Major applications of semiconductor lasers include light sources for magneto-optic data storage, compact disc players, printers, and optical fiber telecommunications.
Commercially-used Al.sub.x Ga.sub.1-x As and In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y semiconductor lasers are conventionally tuned by varying temperature and operating current. However, these tuning techniques are relatively limited in attainable tuning range and thus do not fully exploit the available semiconductor gain bandwidth. In addition, mode hopping tends to create difficulties in reaching a specified wavelength, and the tuning may therefore be discontinuous. To achieve greater tunability across a wide gain bandwidth of a semiconductor medium, external cavity lasers (ECLs) have been employed. A tunable ECL contains an optical gain medium (i.e., a laser semiconductor diode with antireflection coatings on one or both facets), optics for coupling the output of the gain-medium waveguide to the free-space mode of the external cavity, one or more wavelength-selective filters, and one or more mirrors for defining an external feedback path. Various methods of delineating the laser optical resonance cavity are available. They include reflective optics such as prisms, gratings and other dichromic filters - which are narrow-band reflectors, and reflective coatings and dielectric mirrors which are capable of acting as broadband reflectors. Tuning of an ECL is achieved by either altering the characteristic reflection wavelength of the end reflectors (which in turn determines the resonating wavelength inside the cavity), or by changing the length of the laser cavity itself. To reduce mode hopping the wavelengths of the end reflectors and of the laser cavity resonance are advantageously changed proportionately. This proportionate change is a relatively difficult task, however, and generally involves sophisticated, and expensive, feedback mechanisms. In addition, external cavity lasers generally have lower output power and are typically bulkier than other semiconductor lasers. There has also been recent interest in the development of surface emitting laser diodes, in which the light emerges from the surface of the chip rather than from the edge. This feature allows devices to be packed densely on a semiconductor wafer, with two-dimensional arrays fabricated relatively easily. In some surface emitting laser diodes, the laser cavity is vertical, i.e., perpendicular to the plane of the p-n junction, and such devices are referred to as vertical cavity surface emitting lasers (VCSELs). If VCSELs are able to be made tunable over a broad-range of wavelengths, interest would be even more substantial.
A variety of general techniques exist for tuning such lasers, e.g., by changing the semiconductor energy bands, the dimensions of the laser cavity, or the properties of the reflectors. These techniques include thermal tuning, piezoelectric tuning, electrostatic tuning, and magnetostrictive tuning. Thermal tuning tends to be undesirably slow due to the time required for heat conduction and equilibration, it requires the continuous supply of power to maintain the desired temperature, and it has a limited tuning range because of potential damage to sensitive device components at high temperatures. Piezoelectric actuation, in addition to having a limited tuning range due to intrinsic material limitations, also requires the continuous application of high voltages to maintain the wavelength shift. Electrostatic tuning is widely used in miniature devices employing MEMS (microelectromechanical machine systems) technology (see, e.g., M. Y. Li et al., Electron Lett., Vol. 33, No. 12, 1051 (1997); and M. Y. Li et al., Photonics Tech. Lett., Vol. 10, No. 1, 18 (1998)). Such electrostatic tuning, however, is susceptible to drifts due to accumulation or leakage of electric charge, and also requires the continuous application of relatively high voltages to maintain the state of the device. Magnetostrictive tuning tends to require high magnetic fields to achieve usable strain levels and is also generally non-latchable (i.e., power must be sustained after switching).
Improved tunable laser devices are therefore desired, advantageously a semiconductor laser that is tunable over a relatively large range of the gain bandwidth of the optical cavity medium, and that exhibits latchability.